Dual Solvatochromism of Neutral Red

Abstract

Abstract— The effect of solvent polarity on the electronic absorption and fluorescence properties of neutral red (NR), a phenazine‐based dye of biological importance has been investigated in several neat and mixed solvents. An unusual dual solvatochromic behavior has been observed that reveals the existence of two closely spaced electronic excited states in NR. In low‐polarity solvents the fluorescence of the NR is mainly emitted from the localized excited state, whereas in high‐polarity solvents the emission from the charge transfer state dominates. The dipole moments of the localized and charge transfer states of NR have been estimated from the solvatochromic shifts. The dipole moment of the localized excited state (4.8 D) is only slightly higher than that of the ground state (2.0 D), while that of the charge transfer state is drastically higher (17.5 D). Fluorescence quantum yields and the life‐times of NR have been determined in different solvents and correlated with the solvatochromic shifts.

Solvent Effects on 3-Keto-1H-pyrido[3,2,1-kl]phenothiazine Fluorescence in Polar and Protic Solvents

“…As reported by Han et al, 15 the fluorescence radiative decay rate constants, k r , of PTZ-5 are almost the same in all solvents, but the nonradiative decay constants, k nr , decrease severely from protic solvents to aprotic solvents. According to the discussion on C151 and neutral red, 3,4 it is reasonable to suppose that the emission state of PTZ-5 is always ππ* in all solvents and that the fluorescence quantum yields are modulated by changing the nonradiative decay rate such as C151.…”

“…The fluorescence properties of C151 and neutral red in protic solvents are also affected by specific solvent effects. 3,4 Excitedstate proton transfer (ESPT) is considered a very important reaction in the excited-state dynamics of fluorophore.…”

“…Therefore, the fluorescence quantum yield of neutral red decreased in highpolarity solvents. 4 Specific solvent effects, which are produced by interactions between the fluorophore and neighboring solvent molecules, are more complex than general solvent effects. The fluorescence properties of C151 and neutral red in protic solvents are also affected by specific solvent effects.…”

Indirect consequences of exciplex states on the phosphorescence lifetime of phenazine-based 1,2,3-triazole luminescent probes

“…Related studies indicated that they would interact directly with biocompounds such DNA bases via photoinduced electron transfer. 6 On the other hand, PZ derivatives can present appreciable fluorescence [7][8][9][10] enabling their specific use as fluorescent probes in biological environment, where small quantities are enough to be detected without disturbing the system in study. Some PZ derivatives can also present steady-state phosphorescence, relatively weak but observable at room temperature.…”

A Spectroscopic and Microscopic Study on the Extraction Behavior of a Probe Dye, Neutral Red, by Aerosol OT in the Isooctane/Water System

“…The acid-dissociation constant (K a ) for NR was reported to be 10 -6.8 [12]. Therefore, NR changes to its cationic form (HR + ) under acidic conditions and it shows an attractive interaction with anionic AOT, whereas it changes to its nonionic form, which has a weak interaction with AOT, under alkaline conditions.…”